A Hampden County jury found the defendant guilty of rape of a child with force, in violation of G. L. c. 265, § 22 (a), and indecent assault and battery of a child under fourteen (three indictments), in violation of G. L. c. 265, § 13 (b).1 The defendant timely appealed, and we transferred the case here on our own motion. He argues that a Superior Court judge erred when she denied his motion to suppress evidence obtained from a warrantless search of his apartment, and when she determined that he understood English and was not too incapacitated by alcohol to voluntarily provide a sample of his deoxyribonucleic acid (DNA). He also argues that first complaint testimony admitted at trial impermissibly influenced the jury. Because we conclude that, in the circumstances here, the police possessed sufficient factual information when they determined that a person who appeared to have authority had given consent to enter the defendant’s apartment, and because there is no merit to his other claims of error, we affirm the defendant’s convictions.
1. Motion to suppress. We recite the facts found by the judge at the hearing on the motion to suppress, supplemented by undisputed facts in the record consistent with the judge’s findings.2 *691See Commonwealth v. Butler, 423 Mass. 517, 526 n.10 (1996) (we use uncontroverted facts that do not contradict judge’s findings).
The victim lived in a second-floor apartment with his mother. His grandmother lived in the first-floor apartment of the same building with her boy friend, the defendant. The residents of the upstairs apartment visited the grandmother “on almost a daily basis,” sometimes using a staircase that was inside the first-floor apartment and led to the second floor.
On July 11, 2009, the victim spent the night on a couch in his grandmother’s living room. At approximately 2:30 a.m., the defendant returned to this apartment. The grandmother opened the back door of the apartment and noticed the defendant smelled like alcohol and was “walking wobbly a bit.” After a brief conversation in Spanish, the defendant’s primary language, the grandmother went to bed.
Some time between approximately 2:30 a.m. and 4 a.m., the defendant sexually assaulted the victim on the couch.3 When the defendant “fell asleep or passed out,” the victim went to his grandmother’s room, woke her up, and told her what had happened. After talking with the victim for about twenty minutes, the grandmother telephoned the victim’s mother, who was in her apartment on the second floor, and asked her to unlock the upstairs door so that the victim could return home. The victim went upstairs and told his mother what had occurred.
The mother telephoned the police from her upstairs apartment, giving them the street address of the home, but not an apartment number. The grandmother knew that the police had been contacted and told the defendant to leave the apartment. Several police officers, who knew they were responding to a 911 telephone call alleging a rape, were dispatched to the address.
On their way to the home, two officers saw the defendant walking down the sidewalk, engaged in a short conversation *692with him,4 asked him to sit in their police cruiser,5 and then drove the short distance to the victim’s home. When the mother saw the police, she went downstairs to the porch. The victim also went downstairs and entered the grandmother’s apartment. The mother opened the main door to the building, which led to an entryway with doors for the downstairs and upstairs apartments. The mother led two other officers, who had arrived while the first two officers placed the defendant in the police cruiser, through the open first-floor apartment and into the living room where the victim was apparently waiting. The first officers who arrived at the home noticed that the doors to both apartments were open.
Within minutes after the first two officers were led into the first-floor apartment, other officers arrived and entered the home. In the meantime, the grandmother had moved to the back of her apartment to see her son, who did not live with her, enter the apartment with another police officer.6 When the grandmother returned to the front of the apartment, she found “the house was full of officers.” After speaking with police, the victim was transported by ambulance to a local hospital.
While speaking to the mother and grandmother, the officers learned where the alleged sexual assault had occurred and saw a stain on the couch. Believing the stain to be semen, they wanted to take the cushion cover as evidence. One officer, who came to understand that the grandmother lived in the first-floor apartment, explained “the process [they] were going to go through in collecting evidence” to her, stating, “that if she would allow it [the police] could take the evidence and process it.” The *693grandmother told them they could take the cover “if [they] need[ed] it.”7
The defendant was arrested and taken to the police station. No Miranda warnings were given during the booking process. One of the officers who had been at the scene, and who spoke to the defendant in English, requested a buccal swab and provided the defendant with a saliva sample consent form.8 The officer read the consent form to the defendant in English and told the defendant that “he ha[d] a right to voluntary submission of his DNA swab. We could go forward and attempt to get a warrant for it, and he could deny it, and that it [was] up to him. It [was] voluntary at this time.” During this exchange, the officer stated that the defendant spoke to him in English and appeared to understand everything said to him. The defendant signed the consent form, and the officer witnessed it.9
At the hearing, the defendant argued, as he does on appeal, that where the officers did not conduct a diligent inquiry whether the mother had authority to grant consent to enter the home, the entry was illegal. Therefore, he asserts, the subsequent seizure of a cushion cover from the apartment was also illegal. He also *694argues that his consent to obtain his DNA was not voluntary because he was intoxicated at the time he signed a consent waiver and because he did not understand English. The motion judge, who was also the trial judge, denied the motion.
When reviewing the denial of a motion to suppress, we accept the motion judge’s determinations of the weight and credibility of the testimony, Commonwealth v. Robinson, 399 Mass. 209, 215 (1987), quoting Commonwealth v. Moon, 380 Mass. 751, 756 (1980), and accept the judge’s findings of fact absent clear error. Commonwealth v. Scott, 440 Mass. 642, 646 (2004). “The ultimate legal conclusions to be drawn from the subsidiary findings of fact, however, are matters for review by this court.” Commonwealth v. Robinson, supra. See Commonwealth v. Silva, 440 Mass. 772, 775-776 (2004).
a. Warrantless entry. The defendant asserts that the warrant-less entry by police into his home was not justified because police did not obtain consent from a person with actual or apparent authority. He argues that the judge erred when she found that the police made an understandable and reasonable mistake of fact that the mother could grant entry, and that in accordance with Commonwealth v. Porter P., 456 Mass. 254 (2010), the police were required to ask the person leading them into the home if she lived there.
The Fourth Amendment to the United States Constitution and art. 14 of the Massachusetts Declaration of Rights “expressly provide that every person has the right to be secure against unreasonable searches and seizures in his home.” Commonwealth v. Porter P., supra at 260. See Commonwealth v. Rogers, 444 Mass. 234, 236 (2005). “[A] warrantless entry into a home constitutes a search in the constitutional sense.” Commonwealth v. Lopez, 458 Mass. 383, 391, 394 (2010). “Given the high value that our Federal and Massachusetts Constitutions assign to the warrant requirement, particularly in relation to a dwelling, we impose a heavy burden on the Commonwealth to justify every warrantless search . . . .” Commonwealth v. Tyree, 455 Mass. 676, 684 (2010).
However, “[t]he Fourth Amendment’s proscription of ‘unreasonable searches and seizures’ is not violated when a warrantless entry into a home is based on the consent of a third *695party who the police, at the time of entry, reasonably, but mistakenly, believed had common authority over the premises.” Commonwealth v. Lopez, supra at 393, discussing Illinois v. Rodriguez, 497 U.S. 177, 185-186 (1990). Likewise, a defendant’s right under art. 14 is not violated “if a warrantless search of a home occurs after a police officer obtains the voluntary consent of a person he reasonably believes, after diligent inquiry, has common authority over the home, but it turns out that the person lacked common authority.” Commonwealth v. Porter P., supra at 271. Diligent inquiry consists of two steps: first, “the police officer must base his conclusion of actual authority on facts, not assumptions or impressions,” and second, “even when the consenting individual explicitly asserts that he lives there, if ‘the surrounding circumstances could conceivably be such that a reasonable person would doubt its truth,’ the police officer must make further inquiry to resolve the ambiguity.” Id. at 271-272, quoting Illinois v. Rodriguez, supra at 188.
We take this opportunity to clarify the two-step inquiry required when police enter a home, without a warrant and absent exigent circumstances, after they gain consent from someone they believe to have actual authority over the premises. See Commonwealth v. Porter P., supra at 271-272. We conclude that, in situations such as the one here, police need not conduct a “further inquiry,” the second part of the due diligence analysis, where they possess sufficient facts to form the basis for a reasonable conclusion that a third party has authority to give consent to enter a defendant’s home. See id. (first part of diligent inquiry is that officers’ conclusion must be based on facts, not assumptions). In other words, “police have a duty of further inquiry” when faced with “contrary facts tending to suggest that the person consenting to the search lacks actual authority.” Id. at 272, 273 n.18. Absent those contrary facts or “surrounding circumstances [that] could conceivably be such that a reasonable person would doubt its truth,” id. at 272, quoting Illinois v. Rodriguez, supra, an officer’s conclusion that he has consent to enter a premises, if based on “facts, not assumptions or impressions,” may satisfy the first of the two-part inquiry of due diligence. Commonwealth v. Porter P., supra at 271. See Illinois v. Rodriguez, supra.
“We evaluate the reasonableness of a police officer’s conduct *696based on the information available to him at the time, not on what we later learn to be true.” Commonwealth v. Porter P., supra at 270. “As with other factual determinations bearing upon search and seizure, determination of consent to enter must ‘be judged against an objective standard: would the facts available to the officer at the moment. . . “warrant a man of reasonable caution in the belief” ’ that the consenting party had authority over the premises?” Illinois v. Rodriguez, supra, quoting Terry v. Ohio, 392 U.S. 1, 21-22 (1968). Here, the parties do not contest that the mother lacked actual authority to allow entry into the first-floor apartment.10 Our inquiry therefore hinges on the sufficiency of the facts on which officers relied when they concluded that they had consent to enter the defendant’s home.11 See Commonwealth v. Porter P., supra at 271.
Given the circumstances of this case, the judge did not err when she found there were sufficient facts affirmatively known by the police that justified their conclusion that the victim’s mother had authority to grant entry into the defendant’s home. Police went to the defendant’s home after receiving a dispatch to an address close to the area they were patrolling. The officers knew they were responding to a 911 telephone call concerning the rape of a child. The person who had dialed 911 had given a street address, not an apartment number. The first two officers to arrive at the home were greeted on the porch by a woman who identified herself as the person who dialed 911 and indicated that the child who had been assaulted was inside. The woman opened the front door and led police through two open doors into the apartment where the victim was waiting. While it is best practice for police, prior to entry, simply to ask who is the resident of the home, particularly because “the purpose of the entry does not serve to vitiate the intrusion,” Commonwealth v. *697Lopez, supra at 390, citing Commonwealth v. Forde, 367 Mass. 798, 805 (1975), in this situation, which the judge found to be “an ongoing emergency, public and perhaps medical,” the officers did not make further inquiry because they relied on sufficient, unambiguous facts to conclude that they had consent to enter the premises. Having satisfied the first prong of the diligent inquiry analysis under Commonwealth v. Porter P., supra, they did not need to conduct further inquiry. Cf. Commonwealth v. Lopez, supra (in nonemergency situation, police seeking hotel manager did not gain legal consent to enter from woman not known to police and whose relationship to premises was uncertain); Commonwealth v. Rogers, supra at 239, 241 (ambiguity regarding consent to enter premises existed where officers who arrived unannounced and did not identify themselves asked where they could find defendant, and person who answered their inquiry pointed inside).
b. Seizure of cushion cover. The judge found that, given the legal entry into the grandmother’s home, the seizure was also legal. Because the grandmother, who was the tenant of the first-floor apartment and the person with actual authority, “[did] not give any indication that there [was] a problem with the officers entering,” and “readily agreed” to the seizure of the cushion cover, there was “no infirmity” when the police entered the home and seized the evidence. The judge also found that the officers had “good reason” to ask for the stained cushion cover, which they saw during their “initial emergency investigation” after “hear[ing] the account of the child . . . that a sexual act occurred on the couch.”
Citing Wong Sun v. United States, 371 U.S. 471, 487-488 (1963), the defendant argues that the judge erred when she did not suppress the evidence obtained from the cushion cover as “fruit of the poisonous tree” seized after an illegal entry. However, “[t]he doctrine of the fruit of the poisonous tree or primary illegality, as it is sometimes denominated, is not implicated if the tree is not poisonous.” Commonwealth v. Buchanan, 384 Mass. 103, 108 (1981). We agree with the judge that the entry into the apartment was legal under the doctrine of apparent authority, and thus the seizure of evidence was also legal. Moreover, even if the entry had been illegal, police may gain *698consent to search, as they did here, “from the individual whose property is searched.” Commonwealth v. Lopez, 458 Mass. 383, 392 (2010), quoting Illinois v. Rodriguez, 497 U.S. 177, 181 (1990).
c. Voluntariness of buccal swab. The judge found that the consent to the buccal swab was “free and voluntary.” Although the defendant did not receive the Miranda warnings prior to his signing the consent form, the judge determined that the defendant’s command of English enabled him to understand the form he was signing and that “he was not under the influence of alcohol to the extent he was in anyway [sic] deprived of rational thought, of sensibility, [or] of understanding.” Contrary to the defendant’s assertions, the judge’s findings are supported by the record. See Commonwealth v. Prater, 431 Mass. 86, 89-90 (2000). The grandmother testified that Spanish is the defendant’s primary language, but the judge credited the officers’ testimony that the defendant communicated with them in English, understood their requests made in English, was not too intoxicated to voluntarily sign the consent form, and understood what was being said to him. The judge “was satisfied beyond a reasonable doubt” that the defendant voluntarily consented and “understood what the form said.” “Questions of credibility are the province of the motion judge who had the opportunity to observe the witnesses,” Commonwealth v. Watson, 455 Mass. 246, 250 (2009). We accept the judge’s findings. See Commonwealth v. Yesilciman, 406 Mass. 736, 743 (1990) (motion judge “entitled to credit the testimony of the police officers over the testimony of the father”).
The defendant concedes that Miranda warnings are not required in these specific circumstances. See Commonwealth v. Costa, 65 Mass. App. Ct. 227, 231-232 (2005). Nonetheless, he argues that without Miranda warnings he was not aware that he could withhold his consent. This argument has no merit. “[Knowledge of a right to refuse is not a prerequisite of a voluntary consent.” Schneckloth v. Bustamonte, 412 U.S. 218, 234 (1973). See Commonwealth v. Rogers, 444 Mass. 234, 237 n.4 (2005), quoting Schneckloth v. Bustamonte, supra at 229, 235 (requirement of “knowing and intelligent” waiver of right to refuse consent would create “artificial restrictions” on police conduct).
*6992. Testimony at trial. The defendant argues that testimony about the victim’s appearance from the mother, who was not the first complaint witness, was an error of law that enhanced the victim’s credibility and resulted in prejudicial error at trial. Because the jury did not convict the defendant of aggravated rape of a child, he claims that the jury may have doubted the victim’s entire account had the mother’s testimony been excluded.
Prior to trial, the parties had stipulated that the first complaint witness was the victim’s grandmother.12 The defendant now objects to the following testimony. The Commonwealth asked the mother, over the defendant’s objection, to describe the victim’s demeanor when he returned to the second-floor apartment. The mother stated that when the victim came upstairs and woke her, he was “pale in the face, like clammy, like he had seen a ghost, like just real clammy and pale in the face . . . kind of like sweaty, [and] startled,” and that this was “unusual” for him. The mother also testified that she telephoned the police, that she saw the defendant in the police cruiser as she approached the ambulance, and that the victim was transported to a hospital.13
“The judge who is evaluating the facts of a particular case is in the best position to determine the scope of admissible evidence, keeping in mind the underlying goals of the first complaint doctrine,” Commonwealth v. Aviles, 461 Mass. 60, 73 (2011), which are to “balance the interest of . . . a complainant (who, as here, may be still a child) in having [his] credibility fairly judged on the specific facts of the case . . . [and in] a defendant. . . receiving] a trial that is free from irrelevant and potentially prejudicial testimony.” Commonwealth v. Arana, 453 Mass. 214, 228 (2009). Generally, it is for the first complaint witness to describe the circumstances surrounding the *700complaint, id. at 221, which may include “observations of the complainant during the complaint; the events or conversations that culminated in the complaint; the timing of the complaint; and other relevant conditions that might help a jury assess . . . the specific defense theories as to why the complainant is making a false allegation.” Commonwealth v. King, 445 Mass. 217, 246 (2005), cert, denied, 546 U.S. 1216 (2006). This doctrine ensures that the defendant is not unfairly prejudiced by repetition of “often horrific details” of an alleged rape, and that the complainant’s credibility is not enhanced unfairly. Id. at 243.
The first complaint doctrine, however, does not “prohibit the admissibility of evidence that, while barred by that doctrine, is otherwise independently admissible.” Commonwealth v. Arana, supra at 220-221 (testimony about victim’s demeanor admissible to rebut defendant’s allegation victim’s complaint was fabricated). See Commonwealth v. Aviles, supra at 75. In determining the admissibility of complaint testimony, judges should also look to “our guidelines for admitting or excluding relevant evidence.” Id. at 73. See Mass. G. Evid. §§ 401-403 (2013). Our review whether the evidence served an evidentiary purpose “separate and apart from the first complaint doctrine,” Commonwealth v. Dargon, 457 Mass. 387, 400 (2010), is to determine whether the judge’s evidentiary ruling was an abuse of discretion. Commonwealth v. Aviles, supra.
The judge did not abuse her discretion when she allowed the Commonwealth to introduce the mother’s testimony concerning the victim’s demeanor. The mother did not testify in any detail as to what the victim told her concerning what had happened in the first-floor apartment. Rather, she described the victim’s demeanor while he was talking to her and recounted her actions of telephoning the police. This testimony was relevant to rebut the defense’s theory, proffered in opening statements and through cross-examination of the victim, that the entire event was fabricated.14 See Commonwealth v. Aviles, supra at 67. See also Commonwealth v. Lawton, 82 Mass. App. Ct. 528, 536-537 *701(2012) (testimony admissible to rehabilitate witness after impeachment evidence); Commonwealth v. Hanino, 82 Mass. App. Ct. 489, 497 (2012) (testimony of officers admissible to explain prior inconsistent statement). The evidence was also used to rebut the defense’s suggestion that the victim did not seem upset when he spoke with his grandmother, who had testified that the victim was “calm” when speaking to her that night. See Commonwealth v. Hall, 66 Mass. App. Ct. 390, 395 (2006) (where defendant raised credibility issue of victim in rape case, Commonwealth could bring other complaint testimony relevant to timing of complaint). Further, the mother’s testimony was relevant to the contested issues of the victim’s state of mind following the crime, see Commonwealth v. Arana, supra at 225, and the timing and circumstances of police involvement in the case. Id. at 227. Because the mother’s testimony did not repeat any details of the event, was relevant, and was not merely cumulative of the grandmother’s testimony, the defendant was not unduly prejudiced. We see no abuse of discretion. See id. at 229.
3. Conclusion. We conclude that the judge did not err when she denied the defendant’s motion to suppress evidence of the buccal swab and cushion cover because the entrance into the home was lawful, the person with actual authority granted consent to take the cover as evidence, and the defendant voluntarily consented to the taking of his DNA. We also conclude that the judge did not abuse her discretion when she allowed the mother’s complaint testimony at trial.
Judgments affirmed.